1. Field of the Invention
The present invention relates to a high electron mobility transistor (HEMT), in particular, a HEMT made of nitride semiconductor materials, and a method of forming the same.
2. Related Prior Arts
A HEMT made of nitride semiconductor materials, typically gallium nitride (GaN), has been well known because those nitride semiconductor materials inherently have wider bandgap energy and show excellently high breakdown voltages. Such a HEMT may be made by epitaxially growing on a substrate, a buffer layer, a channel layer, and a barrier layer. A Japanese patent laid open No. JP 2013-074209A has disclosed a HEMT having, on a silicon (Si) substrate, a buffer layer made of aluminum nitride (AlN), another buffer layer made of aluminum gallium nitride (AlGaN), and a channel layer made of gallium nitride (GaN). The prior art above has also described that the substrate of Si may be replaced by that made of silicon carbide (SiC). Another Japanese patent laid open No. JP 2015-082517 has also disclosed a HEMT having the buffer layer comprising two AlN layers and two AlGaN layers stacked, where the two AlGaN layers have an aluminum composition different from each other.
In a HEMT made of nitride semiconductor materials, an instable phenomenon has been also known where a drain current thereof decreases after turning-off the HEMT, which is called as the current collapsing. That is, after a high drain bias concurrently with a deep gate bias, that is, the transistor is fully turned off under the high drain bias is applied, the drain current does not recover an original value shown before the turning off. This phenomenon may be described as follows: electrons are captured in traps induced within the barrier layer during the turning off, which equivalently charges the surface of the barrier layer in negative and accordingly, narrows the channel. Because the deep traps generally show longer life time, a period from the capture of the electrons to the release thereof, it takes relatively longer time to recover the width of the channel. Thus, the current collapsing degrades high-frequency performance of the HEMT.